1. Field of the Invention
The invention relates to methods and compositions for reducing the viscosity of drilling mud compositions and more particularly relates to the use of humates to thin water-based drilling fluids.
2. Brief Description of the Prior Art
In drilling earth bores or wells, drilling fluids are circulated past the rotary drill bit to remove cuttings, lubricate the bit, seal and cool the bore formation. The procedure is not without certain difficulties. One difficulty is the encountering of certain formations such as gypsum, which "cuts" the drilling fluid to the point that components of the drilling fluid such as bentonite clays will flocculate. The fluid viscosity increases so that there is potential for damage to the rotary drill bit.
Another potential difficulty is encountered when the bore penetrates a heaving shale. A heaving shale absorbs water from the water-based drilling fluids, leaving the swollen bentonite clay component to close the bore around the drill string. Continued circulation of the drilling fluid is impaired and the drill string can seize.
In deep bores, relatively high drilling temperatures; eg. about 300.degree. F. and higher are encountered. In that environment, water-based drilling fluids thicken and can gel. This of course impedes pumping of the fluid and increases pumping pressures required to circulate the drilling fluids. In some cases, circulation may become impossible. This high temperature instability of the water-based drilling fluid or mud may even be aggravated in the presence of fluid contaminants such as gypsum, salt, cement and like contaminants.
Heretofore, a wide variety of drilling fluid additives have been proposed to stabilize and/or reduce the viscosity of water-based drilling muds. Included in the list of such additives are humates and lignitic materials. Commercially, leonardite, believed to be a naturally oxidized lignite, has been used as a drilling fluid additive to stabilize and thin drilling fluids. Leonardite imparts high temperature stability to water-based drilling muds, particularly if the mud is thinned with a lignosulfonate additive.
We have now discovered that a particular, naturally occurring, humate composition of a unique compositional make-up related somewhat to but different from leonardite, will under some conditions, impart even greater high temperature stability to water-based drilling muds than previously obtainable with leonardite. In addition, on an equal dry weight basis the composition employed in the method of our invention is the equal of, or superior to, leonardite, and in some instances lignosulfonates, as viscosity reducing additives in water-based drilling fluids. Other advantages of the invention will be described more fully hereinafter.